The disclosures herein relate in general to electronic circuitry, and in particular to a method and circuitry for a relaxation oscillator with current and voltage offset cancellation.
A relaxation oscillator is a well-known circuit that generates a periodic non-sinusoidal output signal, such as square and triangular waveforms. Relaxation oscillators are used in many applications, such as computer applications where the square wave is used to provide a clock signal.
Relaxation oscillators are commonly implemented as a resistor-capacitor (RC) circuit in combination with a threshold sensitive device. A current flowing through the resistor charges up the capacitor. The threshold sensitive device does not respond to the voltage on the capacitor until the voltage on the capacitor exceeds the threshold of the device, at which time the device turns on and discharges the capacitor. This process is then continuously repeated as long as the circuit is energized.
One problem with relaxation oscillators is that the oscillation frequency of a relaxation oscillator is strongly temperature dependent. Relaxation oscillators commonly include two current sources: a first current source that outputs a first current, and a second current source that outputs a second current. In an ideal relaxation oscillator, the first current and the second current are exactly equal when there is a constant temperature, and change by the same amount as the temperature varies.
In actual practice, however, the first current and the second current are often slightly different when the temperature is constant, and change by differing amounts as the temperature varies. The first and second currents are said to be different by a current offset, even when temperature does not change. The current offset is a statistical value that results, in part, from transistor mismatch. (Transistor mismatch is the difference between transistors, which are intended to be identical, that results from, for example, random variations in the fabrication process.)
The current offset is strongly temperature dependent in that small changes in temperature can lead to substantial changes in the difference between the first and second currents. The oscillation frequency of the resonant oscillator, in turn, varies in response to the differences between the first and second currents, which vary strongly with temperature.
Another problem with relaxation oscillators is that relaxation oscillators frequently suffer from phase noise at low offset frequency. Relaxation oscillators commonly include a comparator that has a positive input terminal and a negative input terminal. In an ideal relaxation oscillator, the comparator detects the moment that the voltage on one input terminal exceeds the voltage on the other input terminal.
In actual practice, however, there is a slight difference between when the comparator detects the voltage on one of the input terminals exceeding the voltage on the other input terminal, and when the voltage on the one input terminal actually exceeded the voltage on the other input terminal. The difference in timing is said to be due to an offset voltage which causes the comparator to trigger at a time that is slightly different from when the event actually occurred. The offset voltage is a non-ideality of the comparator that results, in part, from transistor mismatch. The exact amount of the offset voltage is unpredictable because it is a statistical value. Since the offset voltage alters the timing of the comparator, the offset voltage introduces phase noise at low offset frequency.